var stream = require('stream');
var Readable = stream.Readable;
var rs = new Readable();

var c = 'a'.charCodeAt(0); //返回其Unicode码
/**
 *String.fromCharCode()方法将Unicode编码转变为字符
 *
 */
rs._read = function () {
    // console.log(c);
    if (c <= 'z'.charCodeAt(0)) {
        rs.push(String.fromCharCode(c++));
    } else {
        rs.push(null); //结束添加
    }
}
//!将可读流输出一个可写流
rs.pipe(process.stdout);
//process.stdout向命令行屏幕上打印一些数据